1. Field of Invention
The invention relates to the adjustment of ink ejection timing of an ink jet printer.
2. Description of Related Art
Known ink jet printers print on a recording sheet when the capacity of each ink reservoir chamber formed in an actuator of a print head is expanded and restored using piezoelectric elements, so that the ink in the reservoir chamber is pressurized and therefore ejected out of the ink ejecting pores formed in the ink reservoir chamber onto a recording sheet. A print head is normally mounted on a carriage for reciprocating movements in the direction along the width of a recording sheet. While being reciprocated, the print head ejects ink to form a print image on a recording sheet.
In an ink jet printer as described above, the ink ejecting pores are disposed at a predetermined distance apart from the plane of a recording sheet. Therefore, when an ink droplet is ejected out of an ink ejecting pore upon application of a drive voltage to the corresponding piezoelectric element, it takes a predetermined transit time for an ink droplet to land on a recording sheet. Consequently, in order to cause an ink droplet to land at a predetermined position on a recording sheet, it is necessary to apply the drive voltage to the corresponding piezoelectric element at a time that takes into account the transit time. The transit time of an ink droplet depends on the ejection speed of the ink droplet, the size of the gap between the ink ejecting pores and the plane of a recording sheet, and the like. Based on these factors and the moving speed of the print head mounted on the carriage, the time of ejecting an ink droplet can be determined. In other words, if the aforementioned quantities are estimated, the time for applying the drive voltage to an ink piezoelectric element can be determined so that an ejected ink droplet may land on a predetermined position on a recording sheet.
In the conventional ink jet printers, however, the displacement characteristic of the piezoelectric elements deteriorates over long periods of use so that the pressurization of the ink becomes insufficient, resulting in an undesirably reduced ejection speed of the ink droplets. The ink droplet ejection speed may also decrease due to deformation of component members of a print head, such as a diaphragm and the like, over long periods of use. Such a reduction in the ejection speed increases the ink droplet transit time so that the actual landing positions of ink droplets on a recording sheet deviate from the intended landing positions. Moreover, if a print head on the carriage reciprocates while ejecting ink droplets, a reduction in the ink droplet ejection speed results in deviations from the intended landing positions in opposite directions corresponding to the moving directions of the print head. In this case, a longitudinal line (in the paper-conveyance direction) printed on a recording sheet becomes rough or disturbed, thereby significantly degrading the print quality.